1. Field of the Invention
The present invention relates to an image forming apparatus and a process cartridge using a contact type charging system in which a charging device charges a desired body in contact with the body, and a developer for use in the contact type charging system.
2. Description of the Background Art
In an electrophotographic image forming apparatus, a charging means for charging an image carrier, e.g., a photoconductive element to preselected potential has traditionally been implemented by a corona charging system. A corona charging system includes a wire electrode or similar discharge electrode and a shield electrode surrounding it and applies a high voltage between the discharge electrode and the shield electrode, so that the resulting corona shower charges the image carrier to preselected polarity.
Today, a contact type charging system is replacing the corona charging system because it produces a minimum of ozone and consumes a minimum of power. In a contact type charging system, a charging member is held in contact with the image carrier and applied with a preselected bias for charging the surface of the image carrier to preselected potential. This type of charging system uses a charge roller, fur brush, magnet brush, blade or similar charging member.
A contact type charging mechanism is a mixture of a discharge charging mechanism and an injection charging mechanism, as known in the art. The discharge charging mechanism charges the surface of the image carrier by using discharge to occur in a small gap between a contact type charging member and the image carrier. In this charging mechanism, a discharge start voltage necessary for discharge to start in the above gap exists, so that the charge potential of the image carrier is not proportion to the value of the bias, but proportional to the value of “bias—discharge start voltage”. More specifically, the bias to be applied must be higher than the resulting charge potential. Further, the above charging system produces discharge products although smaller in amount than the corona charging system. The discharge products deposit on the image carrier and bring about various problems including the run of a latent image formed on the image carrier.
On the other hand, the injection charging mechanism causes a charging member to inject charges into the image carrier to thereby charge the surface of the image carrier. More specifically, charges are injected into a trap level present on the image carrier surface or conductive grains present in a charge injection layer or similar charge holding member. The injection charging mechanism, therefore, does not need discharge and establishes potential proportional to the bias on the image carrier. More specifically, this charging mechanism can charge, even if the voltage applied to the charging member is lower than a discharge threshold, the image carrier to potential corresponding to the voltage applied. In addition, because discharge does not occur, the problems ascribable to discharge products and including the running of a latent image are obviated.
The image carrier for use in the injection charging mechanism will be described more specifically hereinafter. The image carrier for this mechanism should preferably include a surface layer whose volumetric resistance is between 1010 Ω·cm and 1014 Ω·cm. While such an image carrier may be implemented as an amorphous silicon photoconductive element having volumetric resistance of about 1013 μ.cm, an electrophotographic photoconductive element provided with an injection layer on its surface is also preferable from the resistance adjustment standpoint. More specifically, there has been proposed to form a charge injection layer with fine conductive grains dispersed in resin on the surface of an inorganic photoconductive element or a split-function type organic photoconductive element or to disperse conductive grains in a charge transport layer for thereby causing the charge transport layer to bifunction as a charge injection layer.
As for the charge injection layer, light-transmitting resin with high ion conductivity may be mixed or polymerized with an insulative binder, or medium resistance, photoconductive resin may be used alone. It is, however, a commoner practice to form, on the outermost surface of the image carrier, a resin layer in which conductive grains implemented by a metal oxide are dispersed. In this structure, charges can be injected into the conductive grains present on the surface of the image carrier, realizing injection charging. In addition, the insulative binder obstructs the migration of charges between the conductive grains to thereby reduce the run of a latent image. Let the conductive grains contained in the surface layer and exposed on the surface of the image carrier be referred to as subject conductive grains hereinafter.
The prerequisite with the injection charging mechanism is that, to enhance injection efficiency, the charging member and image carrier desirably contact each other, i.e., the charging member surely contacts one point of the image carrier. Particularly, when the surface layer of the image carrier is formed of resin containing the subject conductive grains, injection charging is effected with the charging member and subject carrier grains contacting each other, so that the charging member must contact the exposed, subject conductive grains with high probability.
When one point of the image carrier contacts only one point of the charging member in the region where the charging member and image carrier contact each other, it is necessary that the image carrier and charging member surely contact each other for even charging. However, it is difficult for a charge roller, fur brush, magnet brush, blade or similar conventional charging member to surely contact the image carrier due to limited machining accuracy and the shave-off of the image carrier ascribable to aging.
If the charging member can contact one point of the image carrier at a plurality of points, then the probability of contact is enhanced. A typical method for implementing this configuration is providing a difference in moving speed between the image carrier and the charging member at the point of contact. However, it is difficult for a charge roller to contact the image carrier with a great speed difference because of friction to act between the roller and the image carrier. A fur brush, a magnet brush or similar contact type charging member can relatively easily contact the image carrier with a speed difference. However, although a fur brush is flexibly deformable and can desirably contact the image carrier, there arises a problem that, e.g., carrier grains released from the charging member toward the image carrier enter a developing device.
To improve electrical contact of the contact type charging member and image carrier, Japanese Patent Laid-Open Publication No. 10-307454, for example, proposes to cause conductive grains to intervene between the charging member and the image carrier, so that a speed difference can be easily established particularly when use is made of a charge roller. The conductive grains, intervening between the contact type charging member and the image carrier, will be referred to as charge-promoting conductive grains hereinafter. Functions unique to the charge-promoting conductive grains will be described hereinafter.
The charge-promoting conductive grains may be directly fed to charging means, as taught in the above Laid-Open Publication No. 10-307454, or may be fed from developing means, as taught in Japanese Patent Laid-Open Publication No. 2000-81771, or may be fed from an image transferring section, as taught in Japanese Patent Laid-Open Publication No. 2001-242686. In any case, the conductive grains are conveyed to a charging position where the image carrier and contact type charging member contact each other, and deposit on the charging member.
Even when the surface of the charging member or that of the image carrier is not uniform, the charge-promoting conductive grains thus held at the charging position fill up gaps and improve electrical connection. Further, such conductive grains play the role of a spacer that allows the charging member and image carrier to contact each other with a speed difference. In this manner, the conductive grains maintain the charging member in close contact with the image carrier, so that the charging member can desirably charge the image carrier by injection charging.
When the image carrier is chargeable to negative polarity, the charge-promoting conductive grains can electrostatically deposit on the image carrier if they are implemented by an n type semiconductor or if a p type semiconductor is contained in the surface of the image carrier. When the image carrier is chargeable to positive polarity, the conductive grains can deposit on the image carrier if they are implemented by a p type semiconductor or if an n type semiconductor is contained in the surface of the image carrier. This is presumably because friction acts between the charging member and the image carrier due to the speed difference and generates heat that causes electrons in the semiconductor to migrate, so that the conductive grains are charged to polarity opposite to the polarity of the image carrier.
Functions available with the charge-promoting conductive grains at positions other than the charging position will be described hereinafter on the assumption that the conductive grains are fed from developing means together with toner grains.
The charge-promoting conductive grains are released from the charging member to the image carrier and then transferred from the image carrier together with the toner grains at an image transfer position, so that the amount of the conductive gains at the charging means decreases little by little. It is therefore necessary to adequately replenish charge-promoting conductive grains for insuring expected injection charging for a long time. While various methods are available for replenishing the conductive grains, as stated earlier, feeding them from developing means together with toner grains, among others, is preferable because no exclusive feeding means is required.
As for replenishment from the developing means, the charge-promoting conductive grains exist in the developing means as part of a developer, which is toner in the case of a single-ingredient type developer or a toner and carrier mixture in the case of a two-ingredient type developer. When the developing means develops a latent image formed on the image carrier, the conductive grains are transferred from a developer carrier to the image carrier in an adequate amount together with toner grains. The resulting toner image is electrostatically transferred from the image carrier to a sheet or recording medium or an intermediate image transfer body at the image transfer position. At this instant, although the toner grains are easily transferred by being pulled toward the sheet or the secondary image transfer belt, the conductive grains are not done so, but are partly left on the image carrier. In a cleanerless, image forming apparatus not having a cleaning member between the image transferring means and the charging means, when image formation is repeated with the image carrier, the toner grains and conductive grains left on the image carrier after image transfer are again conveyed to the charging means by the image carrier.
The residual toner is conveyed via the charging position by the image carrier or is released from the charging member to the image carrier little by little and then brought to the developing position and collected there. The charge-promoting conductive grains left on the image carrier are also conveyed to the charging position by the image carrier and deposit on the charging member to promote injection charging. Thereafter, such conductive grains are released from the charging member to the image carrier later and then conveyed to the developing position by the image carrier. At the developing position, while the residual toner grains are easily collected by a bias electric field for development, the conductive grains are not done so because of conductivity. As a result, although part of the conductive grains is collected, the other conductive grains remain on the image carrier. In this manner, the conductive grains, remaining on the image carrier, serve as a spacer between the toner grains and the image carrier, promoting efficient image transfer at the image transfer position and enhancing efficient toner collection at the developing position.
As stated above, the charge-promoting conductive grains effectively function in each of the charging, developing and image transferring steps.
As for the charge-promoting conductive grains, some different studies on grain size have been reported in the past. Japanese Patent Laid-Open Publication Nos. 10-307454 and 2000-81766, for example, propose to use zinc oxide grains, which are an n type semiconductor, having a mean grain size of several micrometers. At the same time, the above documents describe that the charge-promoting conductive grains may be present not only in the form of primary grains but also in the form of a cohered mass of secondary grains, i.e., configuration is not important so long as the functions of the conductive grains are achievable.
Japanese Patent Laid-Open Publication No. 2001-235891, for example, studies the grain size of the charge-promoting conductive grains more specifically and teaches the following. The conductive grains exist in the form of a cohered mass of primary grains having a number-mean grain size of 50 nm to 500 nm, contain at least the cohered mass of primary grains whose grain size is 1.00 μm or above, but below 2.00 μm, and has the content of the cohered mass of primary grains whose grain size is 1.00 μm or above, but below 2.00 μm, confined in a preselected range. The above document describes that such conductive grains do not easily, firmly adhere to the surfaces of toner grains, can be fed to the image carrier in a sufficient amount during development, easily part from the surfaces of the toner grains during image transfer, can be efficiently fed to the charging position via the image carrier after image transfer, exist at the charging position in a uniformly scattered condition, and can be stably held at the charging position.
Further, Japanese Patent Laid-Open Publication No. 2001-235896 pays attention to a problem that, among the charge-promoting conductive grains, grains with extremely small grain sizes tend to firmly adhered to the surfaces of residual toner grains and lower the chargeability of the residual toner grains collected in the developing step. To solve this problem, the above document proposes to confine the amount of the conductive grains whose grain size is 0.5 m or below in a particular range.
It is to be noted that a grain size to repeatedly appear herein refers to a number-mean grain size.
However, experiments showed that when the charge-promoting conductive grains held at the nip between the image carrier and the contact type charging member were continuously used, they caused an image to run. By analyzing the surface of the image carrier after the running of an image, we found that the conductive grains formed an aggregate and adhered to the surface of the image carrier, and detected, by analyzing the conductive grains, nitric acid ions. This will be described more specifically hereinafter.
Even the injection charging mechanism causes discharge to occur, if a little, for the following reason. Because the resistance of the image carrier surface is low and because the resistance of the charge-promoting conductive grains is low, charges are induced on the image carrier surface and cause the dielectric breakdown of an air layer to occur just before the charging member and image carrier contact each other. This easily occurs in a hot, humid environment in which the resistance of the image carrier surface is apt to decrease.
Further, when an AC voltage is superposed on a DC voltage in the injection charging mechanism, the voltage sometimes rises above a discharge start voltage for a moment and causes discharge to occur. As a result, discharge products, including nitrate, are produced and accumulate on surrounding members. If a large amount of moisture is present in the air, then the discharge products react with moisture and exhibit adhesion, as known in the art. More specifically, discharge, if not noticeable, causes the discharge products to accumulate on the charge-promoting conductive grains little by little over a long time to a noticeable amount. The reaction of the products thus accumulated with moisture present in the air results in the cohesion of the conductive grains.
Moreover, the conductive grains are pressed against the image carrier surface by the charging member and therefore firmly adhere to fine dents present in the image carrier surface. Subsequently, the congregate of conductive grains on which the discharge products are deposited grows around the conductive grains so adhered to the dents of the image carrier surface. This phenomenon is generally referred to filming of charge-promoting conductive grains. Because the resistance of the conductive grains is low, an image formed in the portion where filming is present is caused to run, resulting in critically low image quality.
On the other hand, a series of extended studies and experiments showed that the fine, charge-promoting conductive grains not only lower image quality, but also reduce the life of the image carrier, as will be described specifically hereinafter.
When the charge-promoting conductive grains are implemented as a cohered mass of primary grains whose grain size is between 50 μm and 500 μm, as proposed in Laid-Open Publication No. 2001-235891 mentioned earlier, the primary grains are apt to part from the cohered mass due to agitation in the developing device, collision of the conductive grains with each other at the charging position, and friction acting between the charging member and the image carrier. Likewise, even when the grain size of the primary grains is larger than 500 μm, the conductive grains are shaved off due to the occurrences mentioned above with the result that fine powder with a grain size of 1 μm or below is produced. In these circumstances, the absolute amount of fine conductive grains around the image carrier increases little by little due to repeated image formation.
Among the fine conductive grains mentioned above, conductive grains with a grain size of 0.1 μm or below are caused to deposit on the image carrier surface by an adhering force too strong to be coped with by blade cleaning. At this instant, because van der Waals's forces are predominant over an electrostatic force, the above conductive grains adhere not only to portions around the injected conductive grains, but also to the entire image carrier surface. Consequently, a plurality of subject conductive grains are electrically connected together via the charge-promoting conductive grains.
When image formation is repeated over a long time, the image carrier surface is unevenly shaved off due to various causes including friction between the image carrier surface and the charge-promoting conductive grains and additives, and friction between the image carrier surface and carrier grains in the case of the toner and carrier mixture. As a result, the image carrier surface suffers from the maximum irregularity of about 0.6 μm in terms of surface roughness Rz although the irregularity may depend on the image forming process used. It is likely that the conductive grains enter dents so formed in the image carrier surface and therefore adhere to the image carrier even if the grain size is 0.1 μm or above. When image formation is further repeated in such a condition, the conductive grains are continuously subject to a force in the direction of movement of the image carrier at the charging position because, e.g., they contact the image carrier with a speed difference. As a result, the conductive grains are caused to move while shaving off the image carrier surface in the direction of movement of the image carrier surface. Other conductive grains easily enter the shaved portions of the image carrier surface and closely contact the conductive grains already present on the image carrier surface. In this manner, the conductive grains are continuously deposited in the direction of movement of the image carrier surface, electrically connecting the image carrier surface.
The fine, charge-promoting conductive grains thus deposited on the image carrier are not considered to immediately, adversely effect the charging step alone for the following two reasons. First, the charge-promoting conductive grains, like the subject conductive grains, are conductive and therefore do not locally increase resistance when deposited on the image carrier surface. Second, the upper limit of the charge potential at a given point of the image carrier is determined by the bias applied to the charging member and electric resistance between the point where the voltage is applied to the charging member and the image carrier surface, so that the charge potential is not susceptible to the uneven distribution of the conductive substance on the image carrier surface.
However, if the charge-promoting conductive grains deposit on the image carrier over an excessively broad range, irregular charging is apt to occur on the image carrier, depending on the conditions of the charging means. More specifically, in such a condition, a broad conductive region exists and causes the charging member to contact it with higher probability than the other portion. Therefore, if sufficient contact is not established between the subject conductive grains and charging member in the portion where the charge-promoting conductive grains are absent, then charge potential in the portion where they are present is expected to become higher than in the other portion.
The charge-promoting conductive grains additionally function to improve contact of the subject conductive grains and charging member, as stated earlier, so that the contact of the former and the latter varies in accordance with the amount of the charge-promoting conductive grains intervening between them. It is difficult to control the above amount of the charge-promoting conductive grains over a long time. When the amount of the conductive grains decreases due to a long time of operation, irregular charging occurs due to the deposition of the conductive grains, aggravating granularity of an image.
When the amount of the conductive grains decreases, as stated above, there may be executed a procedure that measures or estimates the amount of the conductive grains present with some scheme and increases, if the amount is short, the absolute value of the bias to thereby maintain the charge potential while causing fine irregular charging to evenly occur. However, when the conductive grains deposit over a broad range, only the portion where they deposit maintains expected chargeability. As a result, the above procedure causes the portion where the conductive grains deposit to be excessively charged, resulting in critical irregular charging. Should even charging be maintained to solve such a problem, the amount of the conductive grains present would have to be strictly maintained and would thereby reduce a margin as to the decrease of the conductive grains. In addition, replenishing the conductive grains in such a manner as to strictly maintain the above amount is impracticable without resorting to a highly accurate, expensive sensor.
The fine, charge-promoting conductive grains deposited on the image carrier adversely effect an image although not noticeably effecting the charging step in a short term. More specifically, assume that a boundary between the image portion and the non-image portion of a latent image is present in the portion where the conductive grains deposited over a broad range. Then, electrons are scattered from the non-image portion toward the image portion via the conductive grains deposited, blurring the contour of the latent image.
To describe the above occurrence more specifically, let one of continuous conductive regions present on the image carrier be referred to as an island-like conductive region. More specifically, island-line conductive regions each refer to a particular conductive region electrically connected on the image carrier; the conductive regions themselves are electrically isolated from each other. So long as no charge-promoting conductive grains deposit on the image carrier, the individual subject conductive grain of the image carrier forms a single island-like conductive region. However, when the charge-promoting conductive grains deposit on the image carrier, there occur not only the island-like conductive region of the individual subject conductive grain, but also an island-like conductive region where only the charge-promoting conductive grains deposited and an island-like conductive region where the charge-promoting conductive grains and more than one subject conductive grains contact each other.
To describe the blur of a latent image by using the concept of an island-like conductive region, assume that the area ratio of an image portion included in a single conductive region is A %, that the potential of the image portion is VL (V), and that the potential of a non-image portion is VB (V). Then, the potential of the entire island-like conductive region is expressed as:{VL×A/100+VB×(100−A)/100}(V)
A condition wherein the image and non-image portions exist together in a single island-like conductive region is rare when the conductive region is small, but often occurs as the size of the conductive region increases. Because toner grains are deposited on the image carrier during development, whether the toner grains deposit on the entire island-like conductive region or do not deposit thereon at all is dependent on the image area A mentioned above. The contour of a character image is thickened in the former case or is partly lost in the latter case. In any case, a character image has its edges disfigured while a halftone image suffers from noticeable granularity. Further, when the image area A has a certain value, the potential of the island-like conductive region is likely to substantially coincide with the bias for development and make the development of the above conductive region unstable. This also disfigures a character image or a halftone image.
As stated above, in the system using the charge-promoting conductive grains, the blur of a latent image contour occurs due to the scattering of charges via discharge products. The blur of a latent image contour is similar to the run of an image although the mechanism is entirely different. Particularly, in a hot, humid environment, the deposition of moisture further aggravates such a phenomenon, rendering the blur of the contour more conspicuous.
The charge-promoting conductive grains deposited on the image carrier cannot be easily removed at the image transferring position or the developing position, but remain on the image carrier and continuously blur latent images. A latent image is blurred when its contour is present in the island-like conductive region where the conductive grains already deposited, as stated earlier. Blur also occurs when the conductive grains concentratedly deposit on the contour of a latent image during development, thereby disturbing the contour later.
The blur of a latent image is most conspicuous when the fine powder of the charge-promoting conductive grains whose grain size is two times or more greater than the mean distance between nearby subject conductive grains deposit on the image carrier. While the mean distance between nearby subject conductive grains may be directly measured on a photograph, when uniform dispersion is assumed, the mean distance may be produced by approximation:x×(y/100)(1/3)(μm)where x denotes the mean grain size of the injected conductive grains, and y denotes a volume percent representative of the ratio of the subject conductive grains to the entire surface layer.
Why the fine powder of the conductive grains whose grain size is two times or more as great as the mean distance between the subject conductive grains aggravates the blur of a latent image is as follows. In such a condition, two or more subject conductive grains are electrically connected together with high probability and cause island-like conductive regions to join each other to form a large island-line conductive region, noticeably blurring a latent image. However, when the above mean distance is greater than 0.05 μm, it is presumably difficult for the conductive grains with the grain size two times more greater than the mean distance to adhere to the image carrier surface.
Further, the charge-promoting conductive grains deposited on the image carrier surface not only blur a latent image, but also obstruct image formation by intercepting light.
Moreover, during image transfer that electrostatically transfers toner grains, a strong electric field sometimes appear in a zone (pretransfer zone) upstream of the expected image transfer zone in the direction of movement of the image carrier. More specifically, when the potential of an island-like conductive region is closer to the potential of a non-image portion than to the expected potential of an image portion, a strong electric field sometimes appear at the prenip zone and causes toner grains to fly toward the body to be charged, causing the toner grains to be scattered to thereby aggravate granularity. The scattering of toner grains is particularly noticeable when the charge-promoting conductive grains enter the dents of the image carrier and extend island-like conductive regions in the direction of movement of the image carrier. This problem is more serious in a direct image transfer system configured to directly transfer a toner image from the image carrier to a sheet, because a stronger electric field than in the intermediate image transfer system is used in order to cope with various kinds of sheets different in electric resistance from each other.
As stated above, the fine, charge-promoting conductive grains deposited on the image carrier bring about various kinds of image deterioration. This makes it difficult for the image carrier to preserve high image quality for a long time and thereby reduces the life of the image carrier.
A jumping development system is also known in the art in which the developer carrier and image carrier face each other, but does not contact each other, and toner grains fly between them to develop a latent image. In this system, in particular, it is difficult for the charge-promoting conductive grains to move under the action of an electric field, so that much of them move toward the image carrier by being force by toner grains. Therefore, if various conditions, including the content of the conductive grains, are optimized for feeding a preselected amount of conductive grains in the developing means, then the density of the conductive grains is apt to become higher than in the contact type developing system, blurring a latent image. Another problem with jumping development is that the toner grains are apt to concentrate around the contour of a latent image, causing the conductive grains to also concentrate around the contour and blur the latent image. Such concentrated deposition is likely to thicken or omit the edges of a character image or aggravate granularity of a halftone image before image formation is repeated for a long time. In addition, the island-like conductive regions are apt to join each other and aggravate blur in a long time of operation.
The blur of a latent image ascribable to the charge-promoting conductive grains is more conspicuous when use is made of a charge roller. This is because when use is made of a magnet brush or a fur brush having an extremely large surface area, most fine powder derived from the conductive grains deposits on the brush and deposits on the image carrier little. For the same reason, the blur of a latent image occurs more in the system using the single-ingredient type developer than in the system using the two-ingredient type developer. It follows that blur is particularly noticeable in a system using a charge roller as a charging member and a one-ingredient type developer.
In a cleanerless, image forming apparatus, the charge-promoting conductive grains deposited on the image carrier are not shaved off by a cleaning blade. Therefore, the deposition of the conductive grains becomes critical in a long time. Further, in a developing device of the type superposing an AC voltage on a DC voltage for development, the toner grains and conductive grains hit against each other while moving back and forth in the narrow developing zone and therefore produce undesirable fine powder.